1. Field of the Invention
The present invention relates to a high pressure discharge lamp suitable for a head lamp for an automobile or the like.
2. Related Art Statement
A high pressure discharge lamp with a discharge vessel of quartz has been widely used as a head light for an automobile due to its high brightness and light emission efficiency. The discharge vessel has a light emitting portion and contains a light emitting gas inside of the vessel. The discharge vessel of such discharge lamp is made of quartz and thus transparent, so that the light emitting portion may function as a point light source.
A Japanese patent publication 5-74204A (74204/1993) disclosed a head lamp for an automobile. The lamp has a discharge valve, a vessel for shielding ultraviolet rays and containing the valve, and a reflector. The reflector reflects and projects light emitted by the valve. A Japanese patent publication 5-8684A (8684A/1993) disclosed a head lamp for an automobile having a combination of a metal halide lamp and a high pressure sodium lamp as light sources for the head lamp.
The applicant filed a Japanese patent publication 2001-76677A, and disclosed a high pressure discharge lamp usable as a pseudo point light source for an automobile head lamp. According to the description in the publication, when a light emitter is contained within a light emitting vessel made of quartz and powered, the inner light emitter in the transparent quartz vessel may be shown from the outside of the vessel. The light emitter may thereby function as a point light source. On the contrary, a high pressure discharge lamp using a vessel of a translucent polycrystalline alumina is semitransparent, so that the whole of the vessel functions as an integral light emitter when observed from the outside of the vessel. It is thereby necessary to sufficiently miniaturize the light emitting vessel itself so that the vessel may function as a pseudo point light source. For example, the light emitting vessel has a length of 6 to 15 mm and an arc length in the vessel is 1 to 6 mm. The publication disclosed a novel structure for realizing a high pressure discharge lamp using the light emitting vessel of such a small size.
For example in a head lamp for an automobile, a light emitting vessel is set on a predetermined position. Light emitted from the vessel is then reflected by a reflector to project the reflected light forwardly. The relationship of three dimensional positions of the point light source and reflector, as well as the surface shape of the reflector, are accurately determined, so as to avoid a reduction of condensing efficiency at a focal point. Furthermore, a head lamp for an automobile is operated by switching two lighting modes: running mode and low beam mode. As well known, the head lamp condenses and projects the light beam forwardly in the running mode. The light beam is projected lower in the low beam mode. When a head lamp for an automobile use a high pressure discharge lamp as a pseudo point light source, it is necessary to change the relationship of the positions of the lamp and reflector, corresponding with the different lighting modes, to change the focal point of the projected light beam.
When a light emitting vessel of a high pressure discharge lamp is used as a pseudo point light source, however, it proved to be actually difficult to present an appropriate design satisfying the following two conditions.
(a) To change the relationship between the three dimensional positions of the vessel and reflector to change the focal point of the projected light beam, corresponding to the different lighting modes.
(b) To concentrate the projected light beam at the respective focal points at high efficiencies, corresponding to the respective lighting modes.
The inventor has encountered the following problems. For example, the positions of the light emitting vessel and reflector may be accurately adjusted in the running mode so that the focal point of the light beam is adjusted at a specified point. It is, however, difficult to adjust the projected beam at a specified point by moving the reflector in the low beam mode, according to limitations on the design. This is mainly due to the fact that the vessel is relatively large in size. It is generally effective, for solving the above problems to make the light emitting vessel smaller. As the light emitting vessel is smaller, the production becomes more difficult so that the manufacturing costs may be increased.
When a high pressure discharge lamp using a light emitting vessel made of a translucent polycrystalline alumina is applied for a head lamp for an automobile with a reflector, cracks may be observed in the vessel, after a high energy is supplied to perform lighting cycles of turning-ons and turning-offs over a long period of time. In a head lamp for an automobile using a quartz light emitting vessel, such crack formation are not observed even after electric power higher than a rated voltage is supplied to perform lightning cycles of turning ons and turning offs over a long period of time.
An object of the present invention is to provide a novel high pressure discharge lamp for projecting light and to facilitate the design for improving the condensing efficiency of the projected light at a focal point when the lamp is applied as a pseudo point light source.
Another object of the invention is to provide a novel high pressure discharge lamp having a structure for preventing crack formation in a light emitting vessel after a high energy is supplied to the lamp to perform lighting cycles of turning-ons and turning-offs over a long period of time, when the lamp is used as a pseudo point light source.
The present invention provides a high pressure discharge lamp comprising a light emitting vessel made of a semitransparent ceramic material and having a pair of end portions each with an opening formed in the end portion and a light emitting portion. The lamp further has a pair of discharge electrodes and electrode supporting members each supporting the electrode and fixed to the end portion. An ionizable light emitting substance and a starter gas are filled in the inner space of the vessel. The electrodes are also contained in the inner space. The light emitting portion has a thicker portion and a thinner portion. The thinner portion has a cross sectional area of not smaller than 35 percent and not larger than 80 percent of that of the thicker portion so that the light emitting portion has a brightness center in the thinner portion.
The present invention further provides a head lamp for an automobile comprising the high pressure discharge lamp as a pseudo point light source.
The invention further provides a light emitting vessel for a high pressure discharge lamp. The light emitting vessel is made of a semitransparent ceramic material and has a pair of end portions each with an opening formed in the end portion and a light emitting portion. The light emitting vessel defines an inner space. An ionizable light emitting substance and starter gas are filled in the inner space. The light emitting portion has a thicker portion and a thinner portion, and the thinner portion has a cross sectional area of not smaller than 35 percent and not larger than 80 percent of that of the thicker portion.
The inventor has reached the idea of providing thicker and thinner portions in the light emitting portion and adjusting the cross sectional area of the thinner portion at a value of not smaller than 35 percent and not higher than 80 percent of that of the thicker portion. The brightness center of the light emitting vessel may be thus positioned in the light thinner portion.
That is, when a transparent light emitting vessel such as a quartz tube is used, a light emitter in the light emitting vessel may be observed directly through the transparent vessel from the outside of the vessel. The light emitter may thus function as a point light source. In this case, it is possible to adjust the focal point of the projected and reflected light beam, by adjusting the positions of the light emitter in the quartz vessel and the reflector.
Contrary to this, the inventor has applied a semitransparent light emitting vessel made of a translucent ceramic material so that the whole of the light emitting vessel may function as a pseudo point light source. At the same time, the inventor tried to provide a thinner portion in the light emitting portion of the vessel so that the thinner portion emits more light fluxes than the thicker portion, so that the brightness center is located in the thinner portion. The position and dimension of the thinner portion may be easily and freely selected in the light emitting portion. It is therefore possible to appropriately adjust the position of the brightness center and the distribution of brightness in the light emitting vessel, by appropriately adjust the position and dimension of the thinner portion in the vessel.
A high pressure discharge lamp of the invention may be used as a pseudo point light source to provide a lighting system, light emission from the light emitting vessel may be used for projection. In this case, it is possible to design the position and shape of each optical device on the provision that the position of the brightness center is deemed as a point light source. It is thus possible to facilitate the design of the lighting system, and to improve the condensing efficiency of the projected light beam at a focal point at the same time.
It has been further found that crack formation in the light emitting vessel may be prevented, when the discharge lamp is used as a pseudo point light source, after a high energy is supplied to the lamp to perform lighting cycles of turning ons and offs over a long time period.
These and other objects, features and advantages of the invention will be appreciated upon reading the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art.